Remove PDBFileBuilder::build() and related functions.

PDBFileBuilder supports two different ways to create files.
One is PDBFileBuilder::commit. That function takes a filename
and write a result to the file. The other is PDBFileBuilder::build.
That returns a new PDBFile object.

This patch removes the latter because no one is using it and
in a real life situation we are very unlikely to need it.
Even if you need it, it'd be easy to write a new PDB to a memory
buffer and read it back.

Removing PDBFileBuilder::build enables us to remove other classes
build transitively.

Differential Revision: https://reviews.llvm.org/D26987

llvm-svn: 287697